add new transaction in PRT3

Hi experts.

The problem is such: in transaction PRT3 our customers want to choose fixed assets(FI-AA) for some travel expenses in PR05, and FI document for this expense will be like this:

debit - employee(his vendor in FI system)

kredit - fixed asset.

For fixed assets in my FI system posting key must be 70.

How can I realize it?

I have an idea, but i don't know is it right:1)To implement this posting I wanna create new operation type in transaction PRT3  where I can provide posting keys linked for fixed asset. Because in HRP or HRT there other posting keys? not 70. 2) modify PR05 screen and add field for assets.

3) pass asset throw extension table of bapi BAPI_ACC_EMPLOYEE_PAY_CHECK/POST.

So, How can I add new operation type like HRP or HRT in transaction PRT3?